A known device detects the concentration of particulates in a liquid, for example, the concentration of soot suspended in engine lubricating oil from the light transmittance of the liquid, specifically, the amount of light transmitted through the liquid. The device includes a detection mechanism having a light emission unit and a light reception unit. The light emission unit emits light toward the liquid, and the light reception unit detects the amount of light transmitted through the liquid. Some of the light emitted from the light emission unit is absorbed and scattered by the particulates suspended in the liquid. Thus, the transmitted light amount detected by the light reception unit corresponds to the amount of particulates suspended in the liquid. Accordingly, the particulate concentration may be detected from the transmitted light amount.
In the detection mechanism described above, when a detection surface contacting the liquid that is subject to detection is stained, the detected amount of light decreases. This lowers the detection accuracy of the particulate concentration.
To solve this problem, for example, patent publication 1 describes a device including a detection mechanism arranged in an oil pan for an engine. The detection mechanism includes a float that floats and pitches on the surface of the oil. The pitching motion of the float is used to clean the detection surface of the detection mechanism.
In the device described in patent document 1, the surface level of the oil changes before and after the engine starts running. This produces the pitching motion of the float. Thus, when the amount of the oil in the oil pan decreases, the pitching range of the float is narrowed. This narrows the range of the detection surface that is cleaned. Such a problem that occurs when the oil amount decreases may be solved, for example, by determining the position at which the float is arranged taking into consideration the surface level when the oil amount decreases. However, such a solution would lead to a further problem. Specifically, when there is a sufficient amount of oil, the float would be immersed in the oil and constantly be floating upward. Thus, it would be difficult for the pitching motion of the float to be produced.
In this manner, the prior art device cannot properly remove stains from the detection surface.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 8-86751